1. Field of the Invention
The present invention relates to an acceleration sensor that detects a change in resistance, capacitance, or voltage caused by the displacement of a mass supported by beams to determine acceleration, and a manufacturing method of the same.
2. Description of the Related Art
Acceleration sensors are divided by sensing method into three types: piezoresistive, capacitive, and piezoelectric. For such types of acceleration sensors, there is known a structure in which a mass is supported by beams so that a change in resistance, capacitance, or voltage caused by the displacement of the mass will be detected to determine acceleration. In this structure, the longer the beams, the higher the sensitivity of the sensor. However, as the acceleration sensor becomes smaller in size, there arises a problem that the length of the beams becomes too short for the acceleration sensor to have sufficient sensitivity.
Such an acceleration sensor that determines acceleration based on the displacement of the mass supported by the beams is described in Patent document 1 to be listed below. The acceleration sensor is formed by bonding first and second semiconductor substrates together. On the first semiconductor substrate, the mass, the beams, and a support frame arranged around the periphery of the mass are formed, with one end of each beam secured to the support frame. On the second semiconductor substrate, a weight for increasing the displacement of the mass and a pedestal supporting the support frame are formed. In the structure of this acceleration sensor, the pedestal is formed to have two widths: one larger than the width of the support frame, the other smaller than that of the support frame. The wider part of the pedestal limits the excess downward movement of the mass, and the narrower part limits the excess upward movement of the weight relative to the movement of the support frame, thus controlling the vertical motion of the mass and weight.
In the structure described in Patent Document 1, (See, JP-A-10-048243, Pages 4 to 5, FIGS. 1, 4, and 5) the beams and the support frame are formed in a conventional manner, but the pedestal is formed with partially different widths. However, as the acceleration sensor is micro-miniaturized in this structure, since the length of the beams becomes short, it runs the danger of reducing the accuracy of detection. Further, in the manufacturing method of the acceleration sensor, two semiconductor substrates are processed separately, and then bonded together. Thus the manufacturing processes are complicated and difficult to reduce manufacturing costs.